Electron beam lithography system and method

ABSTRACT

An electron beam lithography system and method which provide an in-plane current density distribution of an electron beam focussed onto a specimen so as to prevent a proximity effect and space charge effect.

BACKGROUND OF THE INVENTION

The present invention relates to electron beam lithography technology tobe applied to LSI manufacturing processes, and to a lithography systemand method aiming at manufacturing fine and highly integrated devices.

Integrated devices such as semiconductor memory devices are becomingfiner and highly integrated more and more. Innovations on manufacturingtechnology, particularly lithography technology, are immense. Theminimum work dimension of recent highly integrated devices is in theorder of sub-micron. Lithography technology which can perform such afine work is electron beam lithography technology. This electron beamlithography technology has a feature that it can process a finer patternthan other lithography technology such as optical type lithographytechnology. A typical example of electron beam lithography technology isdisclosed in Japanese Patent Laid-Open Publication JP-A-59-169131. Asshown in FIG. 1, a conventional electron beam lithography system isconstructed of an electron gun 1 for generating an electron beam, twoapertures 2 and 26 for shaping the beam in the form of square, lenses 3and 5, deflectors 4, 27 and 28, a projection lens 11, and an objectivelens 12. Electrons generated by the electron gun 1 are shaped by thefirst square aperture 2 to obtain a square shaped beam having a uniformcurrent distribution which beam is then focussed on the second aperture26 by the two lenses 3 and 5. The electron beam shaped by the secondaperture 26 of a square shape or any desired shape is focussed onto aspecimen 13 via the projection lens 11 and objective lens 12 to exposeresist on the specimen.

In operation of the above-described conventional electron beamlithography system, an electron beam entered the resist is forwardscattered, and the beam on the substrate surface is back scattered.Therefore, the resist area which should not be exposed is locallyexposed, resulting in a so-called proximity effect. A degree of suchscattering changes depending upon the material of a substrate and apattern density. Therefore, even the patterns of the same design willhave different dimensions after development, depending upon thedensities of adjacent patterns and the substrate material. There is alsoassociated with a problem that if the same pattern is repetitivelydrawn, the dimensions after development become different between thecentral areas and peripheral areas of the pattern. For repetitivepatterns or patterns with various pattern densities used whenmanufacturing highly integrated devices, it is therefore impossible toobtain uniform dimensions as designed. In order to solve the aboveproblems, parameters of pattern data have been conventionally changed ineach lithography operation, such as changing the electron beam exposuretime in accordance with a dimension or a pattern density. With thismethod, however, it is essential that the amount of pattern data isincreased, resulting in a very long pattern preparatory time and drawingtime.

Furthermore, with the above-described conventional electron beamlithography system, a great amount of transmitted electron beam energyis required for drawing a pattern having a large aperture area. As aresult, the focus position of the electron optics displaces from thespecimen surface, giving a so-called space charge effect. With aconventional method, if a large pattern is to be drawn, the pattern isdivided into smaller patterns which are not influenced by theabove-described effect, resulting in a necessity of a long time fordrawing the patterns.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electron beamlithography system and method capable of readily correcting theproximity effect.

It is another object of the present invention to provide an electronbeam lithography system and method capable of readily correcting thespace charge effect.

It is a further object of the present invention to provide an electronbeam lithography system and method capable of drawing a pattern in ashort time.

It is a still further object of the present invention to provide anelectron beam lithography system and method which are less influenced bya pattern density and substrate material.

The above objects can be achieved by providing a certain in-planecurrent distribution of an electron beam to be focussed onto a specimen.

As the means for providing an in-plane current distribution of anelectron beam, there is provided (a) cross striped shielding membershaving a stripe dimension finer than a resolution limit and disposedover the whole surface of, or at a particular area of, each aperturepattern of an aperture plate, or (b) coarse and fine cross stripedpitches in some of the cross striped shielding member having a stripedimension finer than a resolution limit and disposed at each aperturepattern of the aperture plate.

The principle of the present invention will be described below.

Consider the case where a pattern 40 shown in FIG. 2B is drawn by way ofexample. According to a conventional method, exposure is repetitivelycarried out using the apertures shown in FIG. 3. In this case, theaccumulated energy within the resist at the central area and peripheralarea of the pattern has a distribution shown in FIG. 2B taken along aposition line 2--2 of pattern 40. This distribution results from theforward scattering of an electron beam and the back scattering byreflection from a substrate. The distribution changes depending upon theacceleration voltage of an electron beam and the material of asubstrate. The accumulated energy within the resist is given by theproduct of a current density and an exposure time. For a conventionalsystem which has a uniform current density within a beam, it is possibleto solve the above problem by controlling the exposure time. However, inpractice it is impossible to set a drawing time for each pattern.

As shown in FIG. 4, the focus point of electron optics changes with thebeam dimension, i.e., the amount of transmitted current. Therefore,there arises a problem that if the focus point is adjusted using a smallpattern, then blur occurs in a large pattern. It is therefore necessaryto draw a pattern by dividing it into smaller patterns.

In view of the above circumstances, in the present invention, as shownin FIG. 5, cross striped shielding members 530 having a stripe dimensionfiner than a resolution limit are disposed over the whole area of, orover a particular area of, the aperture pattern of an aperture 29.Alternatively, coarse and fine cross striped pitches are provided forsome of the cross striped shielding members. The current density of anelectron beam transmitted through the aperture has a certain in-planedistribution. An in-plane current distribution is set by taking intoconsideration a substrate material, pattern configuration and the likewith aperture 29 so that the above problem can be solved. Aperturepatterns 30, 31, 32, 33, 34, and 35 have uniform in-plane transmittancesof 100%, 90%, 80%, 70%, 60%, and 50%, respectively. It is possible tocontrol the electron beam exposure amount at a wafer by properlyselecting one of these aperture patterns, without changing the currentdensity and exposure time of an electron beam radiated from an electrongun. The proximity effect can be corrected without controlling theexposure time, for example, by using an aperture pattern having a lowtransmittance for a pattern having a high pattern configuration densityon a wafer and by using an aperture pattern having a high transmittancefor a pattern having a low pattern configuration density on the wafer.Furthermore, for a complicated pattern having coarse and fine areaswithin a small area, an aperture pattern 36, 37 or 38 is used which hasa coarse and fine cross striped pitch distribution.

These and other objects and many of the attendant advantages of theinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a conventional lithography system;

FIGS. 2A and 2B show an example of a drawn pattern and the accumulatedenergy within resist at the cross section taken along line A--A';

FIG. 3 is a diagram showing an aperture pattern;

FIG. 4 illustrates a blur amount caused by the space charge effect;

FIG. 5 is a plan view of an aperture which provides various currentdistributions;

FIG. 6 is a schematic diagram of a lithography system according to thepresent invention;

FIGS. 7A to 7E are cross sections illustrating aperture formingprocesses;

FIGS. 8A and 8B are plan views showing examples of patterns to be drawn;

FIGS. 9, 14 and 19 are schematic diagrams showing lithography systemsaccording to the present invention;

FIGS. 10 and 15 are cross sections of substrates to be worked, whichhave a polysilicon film and a tungsten film, respectively;

FIG. 11 is a graph showing a transmittance distribution within anaperture pattern;

FIGS. 12A, 12B and 13 are plan views showing apertures and patterns tobe drawn;

FIG. 16 shows a pattern layout of a 64M bit DRAM;

FIG. 17 is a graph showing a distribution of line widths within a memorymat;

FIG. 18 is a graph showing a distribution of line widths after exposurecorrection;

FIG. 20 shows a pattern layout of a third aperture;

FIG. 21 is a diagram showing an example of area division of a memorymat; and

FIGS. 22 and 23 are diagrams showing the transmission distribution ofeach square aperture pattern of the third aperture for working apolysilicon film and a tungsten film, respectively, the distributionbeing represented by using contour lines.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1st Embodiment

An embodiment of a variable-shaped type electron beam lithography systemand method will be described, wherein an aperture for controlling acurrent density is provided between first and second square apertures.

FIG. 6 is a conceptual diagram of the system. This system is constructedof an electron gun for generating an electron beam, two apertures 2 and8 for controlling the shape of the electron beam, an aperture 7 forcontrolling a current distribution, a deflector 4 for selecting anaperture pattern of the aperture, a transport mechanism 5, lenses 3 and6, an adjustable lens 9, projection lenses 10 and 11, and an objectivelens 12. The third aperture 7 for controlling a current densitydistribution is disposed over the second aperture 8.

The second aperture 8 includes cell projection patterns 20, 21, 22, and23, and a pattern for a variable-shaped method. A conventional methodusing these patterns can correct the proximity effect only bycontrolling the exposure time.

In contrast, according to the present invention, the proximity effectcan be corrected by controlling the current distribution. First, amethod of manufacturing the aperture 7 for controlling the currentdensity will be described. FIGS. 7A to 7E explain the manufacturingmethod. Resist 50 is coated on the surface of a silicon substrate 49(FIG. 7A). A square mesh pattern or cross striped pattern is formed byan electron beam lithography method (FIG. 7B). After forming thepattern, the surface of the silicon substrate 49 is etched by a depth of20 microns by means of a dry etching method (FIG. 7C). Thereafter, anopening pattern is formed at the bottom surface of the silicon substrate49 (FIG. 7D), and by selective chemical etching, a silicon thin filmarea is formed to obtain a desired aperture 7 (FIG. 7E). Designing thewidth of a stripe of the cross striped pattern will be described withreference to FIG. 6. The width of a stripe is set to 0.1 micron on thesurface of a specimen (in this case, silicon wafer) 13. The electronbeam lithography system shown in FIG. 6 has a reduction ratio of 1/25 .Therefore, the width of a stripe is set equal to or narrower than 2.5micron. The cross striped pitch is made uniform for each of the aperturepatterns 15, 16, 17, 18 and 19. There are prepared six types of aperturepatterns having current transmittances of 50%, 40%, 30%, 20%, and 10%,including the full open aperture pattern 14. The dimension of theaperture pattern is set to 150 microns square larger than 125 micronssquare of the second aperture.

The aperture 7 is disposed over the second aperture 8 as shown in FIG.6. One of the aperture patterns of the aperture 7 is mechanicallyselected by motors 6 mounted on the column.

Consider the case where patterns 41 (5 microns×5 microns) and 42 (7.5micron×0.5 micron) shown in FIG. 8A are to be drawn. According to aconventional method, if the focus point of electron optics is set to asmall pattern equal to or smaller than a 0.5 micron square, blur equalto or larger than 0.2 micron will be generated for a pattern of a 5microns square, as seen from FIG. 4. It is impossible to change a focuspoint for each shot in practice. Therefore, a large pattern is requiredto be divided into a plurality of smaller squares in order to draw apattern while reducing the size of blur to a value equal to or smallerthan 0.1 micron. All patterns are required to be drawn using an electronbeam equal to or smaller than 4 microns square. Therefore, as shown inFIG. 8B, the number of shots becomes twelve.

In contrast, according to the present invention, the aperture pattern 14of the aperture 7 having the transmittance of 100% is selected for thefine pattern 42, and the aperture pattern 17 having the transmittance of50% is selected for the large pattern 41. In the latter case, the totaleffective transmitted current amount is reduced by 1/2, so that bluramount becomes 0.1 micron even for a pattern of 5 microns square as seenfrom FIG. 4. A pattern can be drawn without dividing it into smallerpatterns, while reducing blur amount to a value equal to or smaller than0.1 micron for both wide and narrow beam sizes. The number of shots isfour which is one third of the conventional system. In this manner, thedrawing time can be shortened considerably.

2nd Embodiment

In the first embodiment, the third aperture 7 has been provided. Thesame effect can be obtained by implementing means for controlling anin-plane current density distribution of an electron beam with the firstsquare aperture. An aperture 25 having a plurality of square aperturepatterns shown in FIG. 9 is manufactured by the same method of the firstembodiment. The aperture patterns of this aperture 25 are equivalent tothose of the aperture of the first embodiment. The transmittances of theaperture patterns are set to the same values as the first embodiment. Indrawing the patterns shown in FIG. 8A by the variable-shaped method, thesquare aperture pattern 30 with the transmittance of 100% is selected bythe deflector for drawing the narrow pattern 42 having the side lengthof 0.5 micron, whereas the aperture pattern 35 with the transmittance of50% is selected by the deflector 52 for drawing the larger pattern 41.The same results as the first embodiment were obtained.

3rd Embodiment

An embodiment will be described for drawing the line-space pattern 40shown in FIG. 2B. If this pattern is drawn using a conventionallithography method, the resist dimensions at the peripheral area andcentral area become different from each other even by using the sameamount of applied current. FIG. 2B shows the accumulated energy at thecross section taken along line 2--2 of the line-space pattern. If thispattern is developed, the central two lines can be resolved and theremaining four lines disappear, respectively at a development time B. Ata shorter development time C, although the two lines at opposite endscan be resolved, the central four lines cannot be resolved.

In this embodiment, a lithography system like that shown in FIG. 9 isused. The first square aperture 25 is provided with the aperturepatterns same as the first embodiment, and a plurality of other aperturepatterns to be described below, including the apertures 37 and 36 shownin FIG. 5 and manufactured by the method described with the firstembodiment. The aperture 37 has a transmittance of 50% at the right sideand 90% at the left side, gradually increasing in the left direction.The aperture 36 has a reversed transmittance distribution relative tothe aperture 37. The transmittance change amount depends on theacceleration voltage of an electron beam, a substrate material, and thelike. The transmittance distribution curve indicated by a broken line inFIG. 11 was used in drawing a pattern on a specimen having a tungstenfilm shown in FIG. 10 at an acceleration voltage of 30 kV. The secondaperture 8 is formed with the aperture pattern 39 shown in FIG. 5.

First, the deflector 52 selects the aperture pattern 37 of the firstaperture 25 to focus an electron beam having a current distributiondefined by the aperture 37 onto the aperture pattern 39 of the secondaperture 8 so that the pattern is drawn on the wafer 13. Next, thedeflector 52 selects the uniform aperture pattern 35 having atransmittance of 50% to focus an electron beam having a uniform currentdistribution onto the aperture pattern 39 so that the pattern is drawnby the deflector 53 at the right side of the previously drawn pattern.Lastly, the deflector 52 selects the aperture pattern 36 to draw thepattern at the right side of the pattern drawn immediately before. Inthis manner, the accumulated energy within the resist takes a uniformdistribution on each line over the whole area of the pattern at adevelopment time C as shown in FIG. 2A. Upon development of this wafer,the dimension of each line was obtained as designed.

4th Embodiment

An embodiment will be described wherein aperture patterns for providinga current distribution are formed on the second aperture. FIG. 12A showsa complicated pattern 44. FIG. 12B shows that the shape of a secondaperture 44 and the resist patterns 45 after continuously drawing anddevelopment do not coincide with one another, because of the proximityeffect of the complicated pattern. In order to correct the proximityeffect, aperture patterns 46, 47, and 48 shown in FIG. 13 each havingdifferent cross striped shielding members or meshes 530 are formed onthe second aperture 51. The method of forming these aperture patterns isthe same as the first embodiment. The transmission distribution to becaused by the aperture pattern is designed by simulation using anacceleration voltage of an electron beam, a substrate material, and thelike. The aperture patterns are selectively used to realize a drawnpattern not influenced by the proximity effect. Specifically, theproblem of the proximity effect can be solved by using the aperturepattern 46 or 47 for the peripheral area of the whole pattern layout andthe aperture pattern 48 for the central area of the pattern layout.

5th Embodiment

An embodiment will be described wherein the present invention is appliedto manufacturing semiconductor memories.

In this embodiment, as a wiring pattern for a 64M bit DRAM, the pattern44 shown in FIG. 12A is used, and as a work layer the polysilicon film56 shown in FIG. 15 and the tungsten film 58 shown in FIG. 10 are used.As shown in FIG. 16, the pattern 44 of the 64M bit DRAM is constitutedby a memory mat 60 having a repetition of a same pattern and aperipheral circuit 59 adjacent the memory map. First, there will bedescribed the case wherein a conventional variable-shaped type electronbeam lithography system shown in FIG. 1 is used. In drawing a patternusing this system, the pattern is divided into smaller square patternsso that the number of shots becomes large, increasing the drawing time.Furthermore, the outermost periphery of the memory mat 60 is influencedby the proximity effect. Therefore, the area, inside of the outermostperiphery by 2 to 3 microns, of the resist pattern 45 after developmentbecomes under-exposure than the inner area, so that the resist patternbecomes thinner as shown in FIG. 12B. This phenomenon becomes moreconspicuous if the material of the work layer is made of heavy elementsbecause of greater influence by the back scattering of an electron beam.The resist pattern width relative to the pattern position is shown inFIG. 17, for a polysilicon film having relatively light elements as thework layer, and for a tungsten film having relatively heavy elements asthe work layer. In order to alleviate the influence of the proximityeffect, it is necessary to correct the exposure amount at the peripheralarea. As one method of correcting the exposure amount, there is a methodwhereby the peripheral area is exposed two times. The correctiveexposure dosage at the second time is 10% of the main exposure dosage inthe case of the polysilicon film, and about 30% in the case of thetungsten film. The results of this correction are shown in FIG. 18.However, the exposure time further increases for the both films, taking20 minutes or longer for one chip. In contrast with this, according tothe cell projection method shown in FIG. 14, memory cells for severalbits can be exposed by one exposure so that the drawing time can bereduced to 10 seconds or shorter. However, even with this method, theproximity effect is present as in the above case. Therefore, if thecorrective exposure is performed for the peripheral area by thevariable-shaped method, the drawing time about 30 seconds becomesnecessary which is longer than the main exposure time.

The embodiment according to the present invention will then bedescribed.

The aperture 7 for providing a current density distribution is mountedbetween the first aperture 2 and second aperture 51 as shown in FIG. 19.The arrangement of the third aperture is shown in FIG. 20. A squareopening is formed in the second aperture 5, with cell patterns forrespective layers of a semiconductor memory being disposed in the areawithin the square opening. The third aperture 7 is formed at the centralarea thereof with uniform square aperture pattern 63 having a lowtransmittance, and a plurality of aperture patterns around the pattern63 each having a higher transmittance as the position becomes moreremote from the central area. For example, the square pattern 64 at theupper right has a higher transmission distribution as the position goesfurther toward the upper right. The square pattern 71 at the uppercenter has a higher transmission distribution as the position goesfurther toward the upper center. These aperture patterns are formed byworking silicon in the similar manner as the first embodiment. Selectionof one of the aperture patterns is carried out by electromagneticdeflection.

First, a wiring layer pattern is selected from the second aperture 51 bymeans of an electromagnetic deflector 54. Next, in drawing an area A ofmemory mat 60 shown in FIG. 21, the aperture pattern 64 having thetransmittance distribution as shown is selected by an electromagneticdeflector 6. Thereafter, the electromagnetic deflector 6 sequentiallyselects the aperture patterns 65, 66, 67, 68, 69, 70, 71, and 63corresponding to areas B, C, D, E, F, G, H, and I to draw the patterns.

The transmittance distributions for a polysilicon film and tungsten filmwere used which are shown in FIGS. 22 and 23, respectively.

The dimension variation of the resist pattern obtained in this mannerwas substantially the same within 0.05 micron fluctuation at any pointwithin the memory mat 60. The drawing time was 10 seconds or shorter forone chip.

The influence of the proximity effect differs depending upon thematerial of an upper-layer substrate and a pattern density. Therefore,in order to correct the proximity effect, it becomes necessary to set aproper exposure dosage for each pattern. According to the conventionalmethod, the exposure dosage has been set by controlling an exposure timefor each pattern. Therefore, the amount of pattern data becomes bulky,and the drawing time increases. In contrast, according to the presentinvention, an aperture pattern suitable for correcting the proximityeffect is selected from aperture patterns having various types ofcurrent density distributions, in order to deal with the proximityeffect. It is therefore possible to correct the proximity effect withoutincreasing the amount of pattern data and a drawing time.

Although the present invention has been described as related to anelectron beam lithography system, the same effect can be obtained even acharge particle beam other than an electron beam is used.

It is further understood by those skilled in the art that the foregoingdescription is preferred embodiments of the disclosed system and methodand that various changes and modifications may be made in the inventionwithout departing from the spirit and scope thereof.

What is claimed is:
 1. An electron beam lithography method, comprisingthe steps of:emitting an electron beam in a fixed predetermined dosage;repeating drawing a unit pattern with said electron beam on a substratewherein a plurality of said unit patterns forms a highly integrateddevice pattern; forming said unit pattern by shaping the electron beamwith a first aperture having an aperture pattern matching said unitpattern; and varying a transmittance of the electron beam with at leastone second aperture to selectively control an intensity of an exposuredosage of the electron beam on said substrate.
 2. A method of forming apattern according to claim 1, wherein in said drawing of a unit patternstep that forms a highly integrated device pattern, said highlyintegrated device pattern has central and peripheral areas and said stepof varying a transmittance of the electron beam varies the intensity ofthe exposure dosage of the electron beam within said central andperipheral areas.
 3. A method of forming a pattern according to claim 1,wherein in said varying step a plurality of said second apertures areused, each said aperture having a different intensity distribution.
 4. Amethod of forming a pattern, comprising:coating a resist film on asemiconductor substrate; forming a highly integrated circuit pattern byexposing said resist film repeatedly with a unit pattern in individualshots using energy beam lithography emitted from a source in a fixedpredetermined dosage for each shot; and controlling the transmitted beamenergy with at least one aperture to vary the transmittance for saidshots.
 5. A method of forming a pattern according to claim 4, whereinthe highly integrated circuit pattern has central and peripheral areasand in the step of controlling the transmitted beam energy, thetransmittance is varied to have a high transmittance about saidperipheral area and to have a low transmittance about said central areaof said integrated device pattern.
 6. A method of forming a patternaccording to claim 4, wherein in said controlling step, an electron beamis used as said energy beam and said electron beam is selectively passedthrough one of a plurality of apertures that includes at least a firstaperture having a uniform cross stripe pitch and a second aperturehaving both a coarse cross stripe pitch and a fine cross stripe pitch.7. A method of forming a pattern according to claim 4, wherein furtherin said controlling step said electron beam is selectively passedthrough a first centrally disposed aperture having a low transmittanceand a plurality of additional apertures disposed peripherally of saidcentrally disposed aperture having a transmittance that increases fromlow transmittance in portions near the centrally disposed aperture tohigh transmittance in portions disposed farthest therefrom.
 8. Anelectron beam lithography method, comprising the steps of:forming ahighly integrated circuit pattern on a resist film coated on asemiconductor substrate from a plurality of unit patterns; emitting anelectron beam in a fixed dosage emission of predetermined currentdensity and duration an shaping the beam by passing the beam through afirst aperture having a cell projection pattern in the shape of the unitpattern; repeatedly exposing the coated substrate with said shapedelectron beam; and controlling said exposing by passing said electronbeam through one of a plurality of second apertures to vary an in-planecurrent distribution of said shaped electron beam for each shot inaccordance with predetermined parameters.
 9. An electron beamlithography method according to claim 8, wherein said controlling stepcontrols an in-plane current distribution to be uniform.
 10. An electronbeam lithography method according to claim 8, wherein in saidcontrolling step, said in-plane current distribution is controlled fromone side of said shaped electron beam to the other.
 11. An electron beamlithograph method according to claim 8, wherein said controlling stepcontrols the in-plane current distribution in accordance with a materialof the semiconductor substrate and a pattern configuration of the cellprojection pattern as two of said predetermined parameters.
 12. Anelectron beam lithography method according to claim 8, wherein in saidcontrolling step said electron beam is selectively passed through acentrally disposed aperture having a low transmittance when saidexposing exposes said central area and a plurality of additionalapertures disposed peripherally of said centrally disposed aperturehaving a transmittance that increases from low transmittance in portionsnear the centrally disposed aperture to high transmittance in portionsdisposed farthest therefrom when said exposing exposes said peripheralarea.
 13. An electron beam lithography method, comprising the stepsof:shaping an electron beam emitted from a source in a fixedpredetermined dosage and duration by selecting one of a plurality ofcell projection patterns each having a like unit pattern that forms partof a highly integrated circuit device pattern and each having adifferent transmittance affecting an exposure dosage of the electronbeam; repeatedly exposing a substrate with the electron beam in theshape of the unit pattern, including exposing a first area of the devicepattern with the electron beam shaped by a first one of said pluralityof cell projection patterns providing low transmittance and exposing asecond area of the device pattern with the electron beam shaped by atleast a second one of said plurality of cell projection patternsproviding high transmittance.
 14. An electron beam lithography methodaccording to claim 13, wherein said exposing the substrate step includesexposing a central area of the device pattern as said first area andexposing a peripheral area of the device pattern as said second area.15. An electron beam lithography method according to claim 13, whereinin said exposing step said electron beam is selectively passed throughsaid one of the cell projection apertures that is centrally disposedwith respect to the electron beam that has high transmittance andthrough a plurality of additional cell projection apertures, as said atleast one second cell projection aperture, that are disposedperipherally of said centrally disposed aperture and that have atransmittance that increases from low transmittance in portions near thecentrally disposed aperture to high transmittance in portions disposedfarthest therefrom.
 16. A method of forming a highly integrated circuitpattern having central and peripheral areas on a resist film coated on asemiconductor substrate using electron beam lithography,comprising:emitting an electron beam from a source in a fixed dosage andduration and shaping the electron beam with an aperture defining a unitpattern of said circuit pattern; repeatedly exposing the resist with asingle shot electron beam of fixed dosage shaped by said aperture toform the circuit pattern; controlling the transmittance of the electronbeam after the emitting to have a high transmittance about saidperipheral area of said integrated device pattern and to have a lowtransmittance about said central area of said integrated device pattern.17. A method of forming a highly integrated circuit pattern according toclaim 36, wherein in said controlling step said electron beam isselectively passed through one of a plurality of transmittance affectingapertures having different transmittances, respectively.